The present invention relates to a spectrometer for the spectral analysis of a beam of light radiation, which can be used, for example, on orbiting satellites, on aircrafts, or for other applications.
More particularly, the present invention relates to a spectrometer of the catadioptric type, in other words one based on the use of reflecting and refracting elements.
Catadioptric spectrometers having a variety of configurations and characterized by the use of mirrors and lenses in various configurations are currently in existence. A spectrometer generally has a slit through which an incoming light beam enters, a collimator, a dispersion system and a detector.
The object of the present invention is to provide a spectrometer of the aforesaid type which has a particularly simple structure.
Within the ambit of this general object, the object of a particular embodiment of the invention is to provide a spectrometer in which the glass elements can be reduced toga minimum. A further object of the present invention is to provide a wide-aperture spectrometer.
The reduction of the glass elements is particularly important in spectrometers for space applications, since glass does not withstand cosmic radiation (gamma rays, protons, neutrons). This radiation causes a gradual loss of transparency of up to 70% for virtually all glass, with consequent loss of efficiency of the spectrometer. Furthermore, the xe2x80x9cradiation hardenedxe2x80x9d glass available on the market has low transparency at the short wavelengths (below 500 nm), with the exception of quartz and crown glass.
According to the invention, the collimator of the spectrometer is made with the use of at least one first concave spherical mirror and at least one Schmidt plate. Schmidt plates, in combination with spherical mirrors, are known elements in the construction of what are called Schmidt telescopes. There are at present no known applications of this optical component as a spectrometer.
In one practical embodiment, the plate is positioned between the spherical mirror and the dispersion system. However, the positioning of the Schmidt plate between the two elements forming the dispersion system is not excluded, particularly when the dispersion system comprises, for examples two prisms.
Theoretically, it is possible to construct a spectrometer according to the invention with one spherical mirror and a single Schmidt plate, using a configuration known as xe2x80x9cLittrow configurationxe2x80x9d in the literature. The beam entering through the slit follows an optical path along which it encounters a first portion of spherical mirror from which it is reflected toward the Schmidt plate and passes through this to reach the dispersion system. The dispersed beam emerging from the dispersion system is reflected by a flat mirror and again passes through the dispersion system and then passes a second time through the Schmidt plate, in a portion slightly different from the one passed through initially, and is finally reflected, by a portion of the spherical mirror slightly different from the incoming portion, toward the detector.
A configuration of this type is particularly compact but gives rise to certain problems of mechanical interference, since the slit and the detector have to be positioned on the optical axis of the spherical mirror, between this mirror and the flat surface of the Schmidt plate.
In the most suitable and convenient embodiment of the spectrometer according to the invention, at least the following elements are positioned in succession along the path of the beam from the entry point represented by the slit made in one mirror:
the first spherical mirror,
the first Schmidt plate,
the dispersion system,
a second Schmidt plate,
a second spherical mirror,
the detector.
Optical components can also be placed in suitable positions to eliminate the field curvature and/or the image curvature of the slit (called the slit curvature) as illustrated in greater detail below.
In the absence of correctors of the curvature, the beam entering the device through the slit is reflected by the first spherical mirror toward the first Schmidt plate and emerges from this to pass through the dispersion system. The dispersed beam, emerging from the dispersion system through a second Schmidt plate, is reflected by the second spherical mirror and finally reaches the detector.
Characteristically, and in a different way from what is found in conventional applications, the Schmidt plate, although its axis of revolution passes through the center of the spherical mirror, is struck by the beam off axis.
The dispersion system which is used can be any system suitable for this purpose, for example a system formed by one or more dispersion prisms, one or more dispersion gratings, devices known as xe2x80x9cgrismsxe2x80x9d, or combinations of these elements.
The Schmidt plate can advantageously be made from quartz or radiation-resistant glass of the crown type, as can the dispersion system. Thus all components made from flint glass are eliminated from the spectrometer, providing the advantage of eliminating components subject to loss of transparency as a result of cosmic radiation.
The incoming beam which passes through the slit usually originates from a telescope which typically generates a curved image (field curvature), in other words one not lying on a flat surface. This curvature can be of opposite sign to that introduced by the optical components of the spectrometer. If the two curvatures are identical and of opposite sign, they compensate each other and no further corrective arrangements need to be provided.
Conversely, it is possible that the two field curvatures (that introduced by the spectrometer and that introduced by the telescope which supplies the beam entering the spectrometer) will be of different sizes and will not compensate each other. In some cases, the telescope may supply, a flat image. Depending on the circumstances. It is necessary to provide corrective optical components at suitable positions along the optical path of the spectrometer. These can comprise a curve slit, optical fiber corrective components, corrective elements of the dioptric (lens), catoptric (mirror) or catadioptric (combined) types. These field curvature compensation systems can be associated with the beam entry slit, with the detector, or with both.
Systems for correcting what is called the slit curvature can also be provided.
Further advantageous characteristics of the spectrometer according to the invention are indicated in the attached claims.